In the surgical repair of a diseased or damaged hip by implantation of a joint prosthesis, it is often possible to avoid replacement of the natural acetabulum of the patient. In following this conservative approach, it is highly desirable to select a joint prosthesis that will not give rise in use to excessive wear of the acetabular surface. Additionally, it is highly advantageous if the implanted prosthesis is convertible, without damaging its fixation to the patient's femur, for use in articulatory engagement with an artificial acetabular socket, i.e. for use in a conventional total prosthetic hip, should it become necessary at a future time to replace the natural acetabulum with such a socket.
For the above and other reasons, hip joint prostheses comprising a first member adapted to be implanted in the intramedullary canal of the femur and having a metallic spherical head and a second member adapted to receive and hold said head for mutual articulatory engagement therewith have been developed and widely used in recent years with considerable success. The second member typically comprises an outer metallic shell which fits into the natural acetabular socket and an inner plastic cup which is held inside the outer shell and in turn has a cavity formed therein that is substantially congruent with the head of the first member. The inner cup receives the metallic head through a deformable aperture in the cup, which aperture is in communication with said cavity. Since a substantial portion of the motion of the femur is accomodated in use by articulation of the head of the first member within the cavity of the second member, wear of the natural acetabular surface is minimized. The spherical head of the first member is properly sized for mutual articulatory engagement with an artificial acetabular socket and may be exposed for such engagement, if necessary, simply by removing the second member while the first member remains implanted in the femur. An example of such a prosthesis is disclosed in U.S. Pat. No. 4,172,296.
The two members of the type of joint prosthesis described above are engaged by a "snap-fit" as the head passes through a deformable aperture in the plastic cup. Typically, the surgeon may bring the two members into engagement by compressing the members together with his hands. However, this step may sometimes be difficult because the plastic cup, particularly in the region adjacent the deformable aperture, and the metallic head must be precisely dimensioned to permit engagement of the two members but prevent unwanted withdrawal of the head from the cup. Inevitably, a certain amount of dimensional variation is inherent in the commercial scale production of the two members. Furthermore, engagement by hand of the two members with the first member already implanted in the patient's femur, which will be necessary if the surgeon decides to change the size of the second member to obtain a better fit with the natural acetabulum, can prove to be difficult and awkward.